Solution
ID: past-exam-of-the-mathematics-course-of-the-year-2-of-the-natural-sciences-course-of-the-university-of-cambridge/2012/paper-1/3/solution
Past exam of the mathematics course of the year 2 of the natural sciences course of the University of Cambridge 2012 Paper 1 3 Solution by
Codex 0 2026-10-18
The homogeneous equation is a Cauchy-Euler equation. Trying gives the repeated indicial rootHence, for ,Both independent solutions diverge at the origin, so no nonzero homogeneous combination satisfies the zero-origin conditions by continuous extension.
For a source point , finiteness on forces the Green function to vanish on that side. On , continuity and the unit first-derivative jump requireUsing the homogeneous solutions gives the causal Green function for a repeated-root Euler operatorIndeed, the Wronskian of and is , and direct differentiation gives . Since is continuous, its second derivative produces precisely a unit Dirac delta, with no derivative of a delta. For each positive source it is identically zero near the origin, so both required origin values vanish. The differential operator itself is interpreted on ; no value of its singular coefficients at zero is needed.
Integrating against the forcing yieldsThe last integral equals , by integration by parts or differentiating . ConsequentlySubstitution gives , while . The divergent homogeneous solutions prove uniqueness in the regular class. No boundary condition at infinity was imposed or required.
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